AVL-AthenaVortexLattice-MIT
AVL 3.30 Ur Primer last update 18 Aug 10 Mark Drela, MIT Aero & Astro
Harold Youngren, Aerocraft, Inc.
History
=======
AVL (Athena Vortex Lattice) 1.0 was originally written by Harold Youngren circa 1988 for the MIT Athena TODOR aero software collection. The code was bad on classic work by Lamar (NASA codes), E. Lan and L. Miranda (VORLAX) and a host of other investigators. Numerous modifications have since been added by Mark Drela and Harold Youngren, to the point where only stubborn traces of the original Athena code remain.
General Description
===================
now has a large number of features intended for rapid
aircraft configuration analysis. The major features are as follows:
Aerodynamic components
Lifting surfaces
Slender bodies
Configuration description
Keyword-driven geometry input file
Defined ctions with linear interpolation
Section properties
camberline is NACA xxxx, or from airfoil file
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control deflections
parabolic profile drag polar, Re-scaling
Scaling, translation, rotation of entire surface or body Duplication of entire surface or body
Singularities
Horshoe vortices (surfaces)
Source+doublet lines (bodies)
Finite-core option
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Uniform
Sine
Cosine
Blend
Control deflections
Via normal-vector tilting
Leading edge flaps
Trailing edge flaps
Hinge lines independent of discretization
General freestream description
alpha,beta flow angles
p,q,r aircraft rotation components
Subsonic Prandtl-Glauert compressibility treatment
Surfaces can be defined to "e" only perturbation velocities
(not freestream) to allow simulation of
ground effect
wind tunnel wall interference
influence of other nearby aircraft
Aerodynamic outputs
Direct forces and moments
Trefftz-plane
Derivatives of forces and moments, w.r.t freestream, rotation, controls In body or stability axes Trim calculation
Operating variables
alpha,beta
p,q,r
control deflections
Constraints
direct constraints on variables
indirect constraints via specified CL, moments
Multiple trim run cas can be defined
Saving of trim run ca tups for later recall
Optional mass definition file (only for trim tup, eigenmode analysis) Ur-chon units Itemized component location, mass, inertias
Trim tup of constraints
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遵守秩序level or banked horizontal flight
steady pitch rate (looping) flight
Eigenmode analysis
Rigid-body analysis with quasi-steady aero model
Display of eigenvalue root progression with a parameter
Display of eigenmode motion in real time
Output of dynamic system matrices
Vortex-Lattice Modeling Principles
==================================
Like any computational method, AVL has limitations on what it can do. The must be kept in mind in any given application. Configurations
--------------
A vortex-lattice model like AVL is best suited for aerodynamic configurations which consist mainly of thin lifting surfaces at small angles of attack
and sideslip. The surfaces and their trailing wakes are reprented
as single-layer vortex sheets, discretized into horshoe vortex filaments, who trailing legs are assumed to be parallel to the x-axis. AVL provides the capability to also model slender bodies such as fulages and nacelles
via source+doublet filaments. The resulting force and moment predictions
are consistent with slender-body theory, but the experience with this model
is relatively limited, and hence modeling of bodies should be done with caution. If a fulage is expected to have little influence on the aerodynamic loads, it's simplest to just leave it out of the AVL model. However, the two wings should be connected by a fictitious wing portion
which spans the omitted fulage.
Unsteady flow
-------------
AVL assumes quasi-steady flow, meaning that unsteady vorticity shedding
is neglected. More precily, it assumes the limit of small reduced frequency, which means that any oscillatory motion (e.g. in pitch) must be slow enough
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so that the period of oscillation is much longer than the time it takes
the flow to traver an airfoil chord. This is true for virtually any expected flight maneuver. Also, the roll, pitch, and yaw rates ud
in the computations must be slow enough so that the resulting relative
flow angles are small. This can be judged by the dimensionless
rotation rate parameters, which should fall within the following
practical limits.
-0.10 < pb/2V < 0.10
-0.03 < qc/2V < 0.03
-0.25 < rb/2V < 0.25
The limits reprent extremely violent aircraft motion, and are unlikely to exceeded in any typical flight situation, except possibly during
low-airspeed aerobatic maneuvers. In any ca, if any of the
parameters falls outside of the limits, the results should be
interpreted with caution.
Compressibility
---------------
Compressibility is treated in AVL using the classical Prandtl-Glauert (PG) transformation, which converts the PG equation to the Laplace equation, which can then be solved by the basic incompressible method. This
is equivalent to the compressible continuity equation, with the assumptions of irrotationality and linearization about the freestream. The forces
are computed by applying the Kutta-Joukowsky relation to each vortex,
this remaining valid for compressible flow.
The linearization assumes small perturbations (thin surfaces) and is not completely valid when veloci
ty perturbations from the free-stream become large. The relative importance of compressible effects can be judged by鏖战的意思
the PG factor 1/B = 1/sqrt(1 - M^2), where "M" is the freestream Mach number. A few values are given in the table, which shows the expected range of validity.
M 1/B
--- -----
0.0 1.000 |
0.1 1.005 |
0.2 1.021 |
0.3 1.048 |- PG expected valid
0.4 1.091 |
0.5 1.155 |
0.6 1.250 |
0.7 1.400 PG suspect (transonic flow likely)
0.8 1.667 PG unreliable (transonic flow certain)
0.9 2.294 PG hopeless
For swept-wing configurations, the validity of the PG model
is best judged using the wing-perpendicular Mach number
Mperp = M cos(sweep)
Since Mperp < M, swept-wing cas can be modeled up to higher
M values than unswept cas. For example, a 45 degree swept wing operating at freestream M = 0.8 has
Mperp = 0.8 * cos(45) = 0.566
which is still within the expected range of PG validity
in the above table. So reasonable results can be expected
from AVL for this ca.
如何祛疤When doing velocity parameter sweeps at the lowest Mach numbers,
say below M = 0.2, it is best to simply hold M = 0. This will
greatly speed up the calculations, since changing the Mach number
requires recomputation and re-factorization of the VL influence matrix,
which consumes most of the computational effort. If the Mach number
is held fixed, this computation needs to be done only once.
借调Input Files
===========
AVL works with three input files, all in plain text format. Ideally
the all have a common arbitrary prefix "xxx", and the following extensions:
xxx.avl required main input file defining the configuration geometry xxx.mass optional file giving mass and inertias, and dimensional units xxx.run optional file defining the parameter for some number of run cas
The ur provides files xxx.avl and xxx.mass, which are typically created using any text editor. Sample files are provided for u as templates.
The xxx.run file is written by AVL itlf with a ur command.
It can be manually edited, although this is not really necessary
since it is more convenient to edit the contents in AVL and then
write out the file again.
Geometry Input File -- xxx.avl
==============================
This file describes the vortex lattice geometry and aerodynamic ction properties. Sample input files are in the runs/ subdirectory.
Coordinate system
-----------------
The geometry is described in the following Cartesian system:
X downstream
Y out the right wing
Z up
The freestream must be at a reasonably small angle to the X axis (alpha and beta must be small), since the trailing vorticity is oriented parallel to the X axis. The length unit ud in
this file is referred to as "Lunit". This is arbitrary,
but must be the same throughout this file.
File format
-----------
Header data
- - - - - -
The input file begins with the following information in the first 5 non-blank, non-comment lines:
< | ca title
# | comment line begins with "#" or "!"
0.0 | Mach
1 0 0.0 | iYsym iZsym Zsym
4.0 0.4 0.1 | Sref Cref Bref
0.1 0.0 0.0 | Xref Yref Zref
